High current-carrying-capicity cartridge



March 3, 1964 F. .1. KozAcKA 3,123,694

HIGH CURRENT-CARRYING-CAPACITY CARTRIDGE FUSES WITH MINIMIZED LOSSESFiled Nov. 2, 1961 6 Sheets-Sheet 1 Mach 3, 1964 F. J. KozAcKA HIGHCURRENT-CARRYING-CAPACITY CARTRIDGE FUSES WITH MINIMIZED LOSSES 6Sheets-Sheet 2 Filed Nov. 2, 1961 t im u Izavezzoaf.- Wedebi ellfozaca,

March 3, 1964 F. J. KozAcKA 3,123,594

HIGH CURRENT-CARRYING-CAPACITY CARTRIDGE FUSES WITH MINIMIZED LOSSESFiled Nov. 2. 1961 6 Sheets-Sheet 4 7 lwqrrs S F- IOO 200 400 600Curren'r roring WCJHSBC. level of energy margin of safety absorblnqcop0c|ty amps.

critical current range by )www J'oadzzeg March 3, 1964 F. J. KozAcKA3,123,694

HIGH cuRRENT-CARRYING-CAPACITY CARTRIDGE FUsEs WITH MINIMIZED LossEsFiled Nov. 2. 1961 6 Sheets-Sheet 5 aooo Y |000 5000 omps.X IO

.1a. 250volr, 400 and 600 amps.

2 3 x SYM RMS mp5./

0.ol 4 6 8 lo 2 3 45e eooA max. peck circuit can produce O LDQ'ION Od' NMarch 3, 1964 F. J. KozAcKA 3,123,694

HIGH CURRENT-CARRYING-CAPACITY CARTRIDGE FUSES WITH MINIMIZED LOSSESFiled Nov. 2. 1961 6 Sheets-Sheet 6 q/2 @Cl/2 *fn 16.11. 24, 800 pk.amps.

shorcircuit current voltage across fuse '2 Izavezoad.-

480 It 27,3;1) sym. omperes circuit.

arrasar Patented Mair. 3, 'iS-fl 3 123,694 HIGH CURRENT CRRYING CAPACITYCAR- TRIDGE FUSES WITH MINIMIZED LOSSES Frederick J. Koaacka, SouthHampton, NH., assigner to The Chase-Shawmut Company, Newburyport, Mass.Filed Nov. 2, 196i, Ser. No. 149,554 7 Claims. (Cl. 20th-120) Thisinvention relates to electric high current-carryingcapacity low-Voltagecartridge fuses, i.e. cartridge fuses designed to have a cur-rent ratingof at least 400' amps. at voltages not exceeding 600 volts.

This invention relates more particularly to the type of fuses which areoften designated in the trade as one-time fuses. The term one-time fusehas been coined to distinguish these fuses from fuses the fusibleelement of which can be renewed, and which, therefore, lend themselvesupon renewal of their fusible element to performing a repeatedinterrupting duty. Such fuses are generlly referred to as renewablefuses. The term one-time fuse, as it is widely used in the tradeimplies, however, more than the fact 'that the particular fuse iscapable of interrupting an electric circuit but one single time.Socalled one time yfuses are fuse structures which are less expensive tomanufacture than current-limiting, highinterrupting-capacity fuses andare made of materials whose cost is less than the cost of materials ofwhich current-limiting, high-interrupting-capacity fuses are generallymade.

Current-limiting high-interrupting-capacity `fuses are generally lilledwith quartz sand which is converted into a fulgurite when subjected tothe heat of the arc incident to interruption of a circuit. ln otherwords, the sand is converted into a conglomerate of fused and sinteredquartz particles forming a good conductor of electricity as long as thetemperature of the conglomerate is high. Fuses which are provided with aquartz sand filler, or another pulverulent fulgurite-.tormingarc-quenching filler, must be provided with casings of insulatingmaterials which are heat shock resistant and capable of withstanding forsome time the action of hot fulgurites. Such materials are either of aceramic nature (steatite), or suitable synthetic resin laminatesincluding inserts of woven glass iibers, or glass cloth. Casingmaterials of this description are relatively expensive and not used inmanufacturing one-time fuses within the relatively narrow sense thisterm has acquired in the trade. One-time fuses, within the meaning thisterm is being used in the trade, and is here being used, are fusesadapted to effect but one interruption of a circuit and to be disposedof thereafter, i.e. not renewed, interruption by such `fuses beingeffected by means of a pulverulent non-fulgurite-forming arcquenchingfiller arranged in a tubular casing of a homogcnous organic insulatingmaterial. A characteristic 4feature of all non-fulgurite-forniingarc-quenching fillers such as gypsum, or chalk, resides in the fact thatthey evolve gases `under the heat of electric arcs. For this reason theterms non-iulgurite-forming filler and gas-evolving filler are oftenused as synonyms. The term homogeneous is used in this context in thesense of an antonym of composite synthetic resin laminates includinginserts of woven glass, or glass cloth.

Electric fuses, when carrying current, generate heat, and the Wattlosses occurring in electric fuses, and the heat generated in suchfuses, increase with the currentcarrying capacity, or current rating,thereof. Heat losses are proportional to the second power of the currentcarried by the particular fuse, and watt losses and the concomitant risein temperature become more and more of a problem the higher thecurrent-carrying-capacity, or current rating, of a particular piece ofequipment. The

problem of heat losses becomes rather critical at current ratings of 400amps., and higher ratings. Fuses of relatively high current ratings areoften housed in common enclosures `with other heat generating electricalequipment which adds heat to the relatively large amounts of heatgenerated by one-time fuses of high current-rating.

lt is one object of this invention to provide one time fuses, i.e.non-renewable fuses having pulverulent nonfulgurite-forming gas-evolvingarc-quenching fillers and casings of homogeneous organic insulatingmaterials, which fuses are adapted to carry high currents, yet haveminimal watt losses and operate at relatively low temeratures.

Another obiect of the invention is to provide electric one-time fuseshaving current ratings of at 'least 480 amperes, say 400 or 69()lamperes, which fuses `are less expensive to manufacture than prior artone-time fuses having an equal current rating and are subject to smallerwatt losses than prior art one-time fuses having an equal rating andwhich `fuses operate at considerably lower ternperatures than prior artone-time fuses having an equal current rating.

Another object of the invention is to provide one-time fuses having ahigh current-carrying-capacity, which fuses involve `smaller materialcost than current-limiting highinterrupting-capacity fuses, yet compareperformancewise with current-limiting high-interrupting capacity fusesboth in regard to low temperatures when carrying current, and in regardto interrupting capacity when clearing major faults.

l have made a comparative study of watt losses occuring in fuses havingnon-ferrous terminal elements and in fuses having ferrous terminalelements. The watt losses occurring in the former are less than thoseoccurring in the latter, which is due to the fact that the presence offerrous terminal elements tends to result in eddy currents in theterminal elements and concomitant so-called iron losses which are very.signiiicant when considering fuse structures designed to carry highcurrents such as, for instance, 400' amps., or even higher currents.lron losses are completely avoided in fuses wherein the casing is closedby a pair of terminal plugs of copper, `and the same is true in regardto renewable fuses including terminal elements formed of heavy brassparts. Copper terminal plugs as 4used in current-limiting Iinterruptingcapacity fuses and the heavy brass terminals as used inrenewable fuses are, however, far too expensive to be used in one-timefuses within the restricted meaning this term has been defined above.

This invention is predicated upon the concept that relatively light :andinexpensive terminal caps or ferrules on a non-ferrous metal such asbrass can be used in one-time fuses and thus the occurrence or ironlosses completely eliminated therein, if it were possible to drasticallylimit the pressure occurring in such fuses incident to interruption of afaulted circuit, even in spite or the pressure of a gas-evolvingarc-quenching ller which is necessarily present in such fuses.

Each and every means which tends to limit the generation of pressure ina fuse is conducive to some reduction of the mechanical strengthrequirements of the terminal caps or ferrules thereof. One-time fuses ofrelatively high current-carrying capacity can only be provided withrelatively inexpensive or light terminal caps or ferrules of non-ferrousmetals, and in particular brass, if the fuses include pressure-limitingmeans which are highly effective. Pressure can be limited to some extentby minimizing the mass of metal of which the fusible element or fuselink is made. To this end the fusible elements or fuse link ought to bemade of a metal whose conductivity is high. Copper is a suitable metalsince it has a relatively high conductivity and its cost are relativelyloW. Fuses combining copper links and gas-evolving arc quenching fillerstend to generate very high transient pressures incident to interruptionof a faulted circuit. Pressure generation incident to interruption of afaulted circuit can be further limited by limiting the melting fig-dtand the arcing fiz'dt, by imparting a current-limiting action to thefuse.

Since high current-carrying-capacity current-limiting one-time fuses hadnot been considered to date it became necessary to investigate whethercurrent-limitation is a suiiiciently effective pressure-limiting meansin the presence of a gas-evolving arc-quenching iler to allow on suchfuses the provision of relatively light caps or ferrules of brass, orother nonferrous metal. It was found that current-limiting action is notsuticient in itself, but that the problem can be solved by resorting toa predetermed minimum of current-limiting action.

There are a numbery of ways for defining numerically thecurrent-limiting action of a current-limiting fuse. One characteristicquantity numerically defining degree of current-limitation is thecurrent-limiting ratio. Current-limiting ratio is the ratio of thecurrent required to cause melting of the fusible element or fuse link ofa fuse in 0.01 sec. to the rated current of the particular fuse (seePhilip C. Jacobs, Current-l4imiting Fuses: Their Characteristics andApplications, Technical Paper 56-772, American Institute of ElectricalEngineers). To achieve the ends contemplated by this invention thecurrent-limiting ratio must be less than 30.

In order to achieve this degree of current-limiting, or current-limitingaction, it is necessary, or desirable, to resort to certain structuralfeatures as will be outlined below more in detail.

If the melting fiZ-dl or fusing j`i2-dr is to be minimized withoutresorting to relatively expensive fuse links of silver, the fuse linksought to be of copper. Current ratings of 400 amperes or in excess of600 amperes call for ribbon type fuse links. To maximize the area ofinteraction between the arcs formed incident to interruption and thearc-quenching filler and to thereby minimize the arcing fi2dt and theduration of pressure generation by the arc-quenching filler a pluralityof ribbon fuse links should be arranged in parallel. Apressure-generationlimiting arrangement of two fuse links in parallelwhich is easy to manufacture is obtained by sandwiching the axiallyinner ends of a pair of blade contacts projecting from the outside ofthe casing or fuse tube into the inside thereof between the axiallyouter ends of the ribbon fuse links. In order to avoid thermal damage tothe casing or fuse tube of organic insulating material the rise intemperature of the fuse links while performing their current-carryingduty must be limited. This can be achieved by an overlay of alow-fusing-point link-severing metal, e.g. tin, on each of theaforementioned fuse links. Such overlays are well known in the fuse artand, therefore, do not need to be described in detail in the presentcontext.

The current-carrying capacity of a pair of fuse links arranged to carrycurrents in parallel depends primarily upon the dimensions of the fuselinks, their geometryand the nature (thermal conductivity) of thepulverulent arcquenching filler in which the fuse links are submersed.By controlling these parameters a pair of fuse links can readily headapted to jointly carry continuously currents of at least 400 amperes,say 400 amperes, or 600 amperes. Each of said pair of fuse links shouldhave a plurality of transverse lines of perforations forming zones ofreduced cross-section or cross-sectional area. These zones can readilybe adapted to establish a predetermined currentlimiting ratio. Byadopting an appropriate geometry forthe transverse lines ofperforations, and more particularly for the constituent perforationsthereof, a very high degree of current-limiting action, or relativelylow current-limiting ratio, can be obtained. It is thus readily possibleto obtain current-limiting ratios of less than 30. Such lowcurrent-limiting ratios, i.e. current-limiting ratios which are lessthan 3), result in relatively low pressures during tion;

the interrupting process, even in the presence of pulverulentnon-fulgurite-forming gas-evolving arc-quenching fillers.

For a better understanding of the present invention together with otherand further objects thereof, reference may be had to the followingdescription, taken in connection with the accompanying drawings, and itsscope will be pointed out with particularity in the appended claims:

Referring now to the drawings:

FIG. l is partly a longitudinal section and partly a side elevation ofan Underwriters Laboratories standard size fuse for a 600 volt circuitembodying the present inven- FIG. 2 is a section taken along 2-2 of FIG.l;

FIG. 3 is a section taken along 3-3 of FIG. 2;

FiG. 4 is partly a longitudinal section and partly a side elevation ofan Underwriters Laboratories standard Sile fuse for a 250 volt circuitembodying the present invention;

FIG. 5 is a sectionV taken along 5 5 of FIG. 4;

FIG. 6 is a section taken along 6--6 of FIG. 5;

FIG. 7 refers to a family of fuses and shows watt losses and equilibriumtemperature plotted against current rating;

FIG. o refers to fuses embodying the present invention and is a blockdiagram showing temperatures in degrees C plotted against load inpercent of the rated current;

FIG. 9 refers to fuses embodying the present invention and is a blockdiagram showing watt losses plotted against current rating;

FIG. l0 is a typical diagram showing energy converted into heat in afuse of given design plotted against current in terms of multiples ofthe current rating of the particular fuse;

. FIG. ll shows peak let-through currents of fuses embodying the presentinvention plotted against available currents; i

FIG. 12 shows the time-current curves of two fuses embodying the presentinvention;

FIGS. l3-l5 are diagrammatic representations of ribbon type fuse links;and

FIGS. 16a and 16b are oscillographic records resulting fromshort-circuitrtests of fuses embodying the present invention.

Referring now to the drawings, and more particularly to FIGS. l-3thereof, reference numeral 1,l has been applied to indicate a tubularcasing of an Underwriters Laboratories standard size fuse having avoltage rating of 600 volts anda current rating of 600 amps. Casing 1 ismade of a homogeneous organic insulating material, preferably vulcanizedfiber. Casing 1 contains a pulverulent nonfulgurite forming arcquenching ller Z evolving gas under the heat of electric arcs. Filler 2may be gypsum powder. A pair of blade contacts 3 extends in a directionlongitudinally of casing 1 each projecting from the outside ofV casing 1into the inside thereof. Reference numeral 4 has been applied toindicate a pair of ,ribbon fuse ,links of.copper submersed in filler 2.Fuse links 4 sandwich between the axially outer ends thereof the axiallyinner ends of blade contacts 3. Fuse links 4 are provided with livetransverse lines 4a of circular perforations. The number of perforationsin each line la of perforations exceeds the number of lines ofperforations, i.e. the number ve. The diameter of the constituentperforations of the five lines of perforations 4a is of considerableimportance in order to achieve a sufficiently small melting fi2-dt and asuiiiciently small arcing fz'2dt in spite of the fact that thede-ionizing action of the arc-quenching filler 2 is substantially lessthanl that of quartz sand. Fuse links 4 are adapted to jointly carrycontinuously currents of at least 480 amps. In other words, the fusestructure of FIGS. l-3 has a current rating of at least 400 amps. In theparticular embodiment of the invention shown in FIGS. l3 fuse links iare dimensioned in such a way that the fuse of which they form part isable to carry continuously currents as high as 600 amperes withoutcausing any undue heating, or excessive heating, of its casing l tendingto cause too rapid deterioration of the homogeneous organic insulatingmaterial of which the casing is made. The hottest points in the fuseunder load conditions and overload conditions are the center lines oflinks 4. Each fuse link 4 is provided with an overlay 4b of alow-fusingpoint link-severing metal, eg. tin. When overlays 4 reachtheir fusing point they sever links 4 by virtue of a metallurgicalreaction occurring between the melted overlay metal and the base metal,i.e. the copper of which links 4 are made. This limits the temperatureceiling, or highest temperature, to which links 4 are subjected whencarrying overload currents, and thus limits the highest temperature towhich casing l may be subiected when the fuse care ries overloadcurrents. The constituent perforations of the five lines of perforationsin define five Zones, or transverse lines, or reduced cross-sectionwhere the current density in fuse links 4 is highest. Thecross-sectional area of these five zones of reduced cross-sectiondetermines the melting {i2-dr in terms of ampere square times seconds offuse links 4, and hence also the current-limiting ratio of the fuse.This cross-sectional area is suiiiciently small to result in acurrent-limiting ratio of less than 30, i.e. the ratio of the currentrequired to melt the zones of reduced cross-section in 0.01 sec. to thecurrent rating of the fuse is less than 30. FIG. l2 shows the timecurrent curves of fuses embodying the present invention making it-readily possible to read oil the current-limiting ratio of these twofuses. lt appears from FlG. l2 that the current-limiting ratio is about25, which is a better figure than 30, the latter being about the highestpermissible limit. The number of the zones of reduced cross-section ofthe structure of FIGS. 1 3, inclusive, and the geometry of theconstituent perforations of the lines 4e of perforations determines theclearing ratio ot' the particular link structure when submersed in agiven arc-quenching iller. The clearing ratio is the ratio between themelting fiz'd plus the arcing ft2-dt known as the total clearing ft2-dtto the melting ft2-dt (see above paper of Philip C. Jacobs, In). Theclearing ratio tends to increase with decreasing arc voltage. The higherthe arc voltage and the stabler the arc voltage the smaller the clearingratio. The arc voltage tends to increase with the number of seriallyrelated Zones ot reduced cross-section- An increase of the diameter or"the constituent perforations of the lines of perforations 4a tends toincrease and to steady the arc voltage and to reduce the current ratingof the particular fuse as will be more apparent from what follows.

Pins 5 are formed by spirally wound sheet metal, thus imparting to thesepins resiliency in radial direction thereof. Each pin 5 projectstransversely through one of blade contacts Si and through the casing l,thus supporting blade contacts 3 on casing l. A pair of terminal caps 6of a non-ferrous metal is mounted on the ends of casing l. Each terminalcap 6 defines a passage 6a for one of blade contacts 3. Gaskets 6b of asuitable fibrous material are interposed between the axially outer endsof casing l and the axially inner surfaces of caps d. Substantiallynail-shaped fasteners 7 are driven through the lateral cylindricalsurfaces of caps 6 into the inside of sheet rnetal pins 5 expanding theends of pins 5 situated inside radial bores in casing 1. Fasteners 7thus firmly secure caps 6 to casing l. Caps 6 are mounted suflicientlytightly on casing 1 to substantially preclude the emission ot hotproducts of arcing therefrom formed incident to interruption by the fuseof severe fault currents. The fuse is thus a non-vented fuse, i.e. thereis no pressure relief by venting and, therefore, caps 6 must be able towithstand the transient pressures generated in casing l. duringinterruption of faulted circuits. If the caps ti were mounted relativelyloosely on casing l, thus permitting the escape of arc products, thestrength requirernents placed upon caps 6 would be greatly reduced.

However, the escape of hot products of arcing is a very undesirableoperating characteristic for all indoor applications and shown,therefore, be avoided.

Caps 6 are preferably made of brass and have a wall thickness of lessthan 0.07 inch. Because the thickness of cap-s d is so small and thestrength of the metal of which they are made is small, and since theymust contain virtually all the products of arcing generated in casing lincident to blowing of the fuse, the generation of gases from thegas-evolving arc-quenching ller 2 must be drastically curtailed. Thelatter is achieved by a drastic limitation of the thermal energyreleased in the fuse during an interrupting process.

In FIGS. 4-6 the same reference characters as in FlGS. 1 3 with a primesign added to them 'have been applied. Hence the description of liGS.4-6 can be brief. Casing i is filled with a pulverulent arc-quenchingfiller 2.'. Casing l is made oi a homogeneous organic insulatingmaterial such `as vulcanized fiber and filler 2 is of the gassevolvingvariety, eg. gypsum. Blade contacts 3 extend in direction longitudinallyof casing l from the outside to the inside thereof. Ribbon fuse links 4'of copper are submersed in filler 21. They sandwich between the `axiallyouter ends thereof the axially inner ends of blade contacts 3. Each offuse `links 4' has three transverse lines of perforations, each linecomprising about eight individual pertorations. The above number oflines of perforations and the above number of perforations per line hasbeen round to `be necessary and desirable for circuit volta notexceeding 250 volts where the currents to lbe carried by the fuse andthe current ratings intended to be assigned to the fuse are 4Gb amperes,or in excess of 4G() ampores, say 609` amperes. By giving appropriatedimensions to fuse links 4. a current rating of at least 400 amperes maybe assigned to the fuse of FIGS. 4-6. Lines of perforations da definethree related zones of reduced crosseection which are suiciently smallto result in a current-limiting ratio of less than 30, preferably as lowas Z5. rlhe width of fuse links 4 is only slightly less than the innerdiameter of casing l', and the number of perforations per line 4a',which is about eight, coupled with the number of lines of perforations,which is 3, resuits in a gypsum filled fuse inserted into a circuithaving a circuit voltage of 250 volts in a clearing ratio which, in theaverage, is less than 5. it will be understood that the clearing ratiois aliected by a number of variables, including the nature of thegas-evolving filler 2' provided in any particular instance. The severityof fthe circuit under interruption has also an important bearing on the`clearing ratio. All statements made herein refer 'to 60 cycle A.C. test.circuits having a low power factor and available fault currents up ftoabout 59,00@ arnperes. If a fuse according to the presen-t invention istested in such a circuit its clearing ratio will vary with the faultangle. Considering a large number of tests wherein the fault anglevaries at random, the clearing ratio will vary according to the faultangle :and thus an average clearing ratio will be obtained which isdefined by the aforementioned test conditions. rthis average clearingratio should be less than 5, and preferably less than 4.

Tests carried out with the structure of FIGS. l-4 at 480 volts ratherthan at 600l volts which is the voltage rating of that structurerevealed average clearing ratios between 3 and 4. lf the circuit voltageis raised to 600 volts the clearing ratio increases without exceedingthe upper limit value of 5.

Fl'G. 7 shows the temperature T in deg. C. and the watt loses W of awell designed modern `family of fuses when carrying their rated currentplotted against current rating lr. This family of fuses had terminalelements or a non-ferrous metal and, therefore, the curves shown in FIG.7 are not aiiected by eddy current losses which occur in fuses` havingterminal elements of a ferrous metal, i.e. steel. Though FlG. 7 refersto a family of fuses in which `watt losses are minimized, it is apparentthat watt losses W and temperature T increase significantly withincreasing current ratings. Rise of temperature and Watt losses tend tobecome critical when fuses having a current rating of, or in excess of,400 amps. and having terminal caps, or ferrules, of a ferrous metal, arehoused in an enclosure. Hence it becomes necessary to give increasedattention to watt losses wherever the current rating is in the order, orin excess of, 400 amperes and the fuses are intended .to be accommodatedin an enclosure.

The block diagram of FIG. 8 refers to `a fuse structure substantially asshown in FIGS. 1-3, inclusive, having a current rating of 400i amps. anda voltage rating of 600 volts. This diagram shows temperatures indegrees C. plotted against load in percent of the rated current.Temperatures are indicated for two load currents, namely 100% and 110%of the rated current. The rectangles B indicate temperatures at theblade contacts and the rectangles C indicate temperatures at the centerof the casing of a fuse according to FIGS. 1-3 having terminal caps orferrules of steel. The rectangles B indicate temperatures at the bladecontacts and the rectangles C indicate temperatures at the center of thecasing of a fuse according to lFIGS. 1-3 having terminal caps orferrules of brass. It is apparent from FIG. 8 that the substitution ofbrass for steel goes -a long way in obtaining a cool running fuse.

In FIG. l9 watt losses are plotted against current rating. FIG. 9 refersto a structure of the kind shown in FlGS. l1-3. losses obtaining if thestructure is provided with terminal caps, or ferrules, of brass, whiletl e columns S represent watt losses obtaining if the structure isprovided with terminal caps, or ferrules, of steel. Itis apparent fromFlG. 9 that substitution of brass for steel results in an outstandingperformance improvement.

Electric fuses convert during circuit interruption electric energy intoheat. The design of the particular fuse and in particular the geometryof its fuse link means and the nature of its pulverulent arc-quenchingfiller as well as the nature of the circuit under interruption determinethe amount of heat generated in a given fuse under specified excesscurrent conditions. FiG. 10l shows energy converted into heat in a fuseof given design plotted against current in terms of multiples of thecurrent rating of the particular fuse. The diagram of FIG. 10 is typicalof the operation of any current-limiting fuse.

" There is la vcritical range where energy dissipation or thermalstresses are highest, and this critical range is less than the maximumcurrent which the fuse is'capable of safely successfully interrupting.The heat generated in a fuse is not highest at the highe-st currentswhich the fuse may be called to successfully interrupt but has its peakregion at currents Way below the peak currents which the fuse may becalled to interrupt. `lt is rather difficult to experimentally determinethe pressure prevailing in a fuse casing and acting upon the terminal.caps or ferrules and stressing the latter. However, the energydissipated in the casing, or converted into heat therein, is a fairmeasure of the pressure generated in a fuse casing during aninterrupting process. -As long as a fuse is current-limiting and thepeak of heat generation lies withinV the currentlimiting range of thefuse, the strength requirements of terminal caps, or ferrules, can bereduced significantly by so designing the fuse that its current-limitingaction is substantial.

The pressure which is developed within the casing of a fuse and whichacts upon the terminal caps or ferrules of the latter and tends todeform and damage the same depends upon a number of parameters. Gne ofthese parameters is, for instance, the grain size of the pulverulentarc-quenching filler which is arranged Within the casing of the fuse.Another very important parameter governing internal pressure duringsevere interruptions is the internal volume of the casing and thegeometrical configuration thereof. The aforementioned internal pres- Therectangular columns marked B represent watt sure decreases inversely tothe internal volume. Considering the internal volume of a fuse designedin accordance with the Underwriters Laboratories standards as point ofdeparture, the volume of such a standard fuse may either be decreased orincreased (see Standards for Safety Fuses UL 198 UnderwritersLaboratories, Inc., reprinted `uly 1959). Progressive decrease of thevolume of a standard fuse may result in such an increase of pressureduring severe interruptions that the pressure cannot be contained anylonger by any feasible means. Similarly, progressive increase of thevolume of a standard fuse beyond any feasible dimensions may result insuch a decrease of pressure during severe interruptions that containingthe pressure within the casing does not present a problem any longer.What has been stated above in regard to the objects of this inventionand what is stated below in regard to the means being employed inconnection with this invention refers specifically to UnderwritersLaboratories standards size fuses, i.e. fuses having casings of thesizes defined in the above standards. T he most important dimensionsderived from the above standards defining casing size are length overcaps designated A in FiG. 1, maximum width measured parallel and/ or atright angles to the plane of the blade contacts designated B in FIG. 3,and minimumv thickness of casing wall designated C in FlG. 2. Lengthover caps A is substantially equal to the length of the tubular casing.The values for the quantities A, B and Cas defined above are set forthin the table below:

Dimensions of cartridge-enclosed fuses in inches Electrical rating VoltsAmps. B C

Regarding the relation between the pressure on the walls and the caps ofa cartridge fuse and grain size, there is a definite or critical grainsize resulting in minimum pressure. As a general rule, the grain sizewill be selected with a view to achieving certain electrical performancecharacteristics irrespective of resulting pressure generation.

While the pressure Within a fuse depends upon arc energy, there is nosimple law relating both quantities, pressure depending on arc energy aswell as on the rate at which arc energy is being released.

Static and transient pressures in a fuse can be determined in a numberof ways, yet it is quite difficult to reliably carry out transientpressure measurements. in developing the structures of FIGS. 1-6 nonumerical pressure measurements were made. The fuses were tested as towhether or not the pressure on the caps or ferrules was sufficiently lowto preclude any deformation thereof, and to preclude shearing of thecasing or fuse tube by the pressure transmitted from the caps orferrules through the blade-contact-supporting pins 5 and 5' to thecasing.

Most tests including non-ferrous caps were conducted with caps having awall thickness of less than 0.07 inch.

Y in order to achieve economic designs the wall thickness must bereduced to this order, and the transient pressures within the casingmust be so controlled, or'limited, to allow the use of non-ferrous capswhich are asthin as that. The preferred non-ferrous metal used is redbrass containing copper and 15% zinc. Fuses having a voltage rating of250 volts can be provided with brass caps having a wall thicknesssubstantially less than 0.07, e.g. a wall thickness of less than 0.045inch, both for the 400 amp. and the 600 amp. rating. ln that particularinstance a wall thickness of 0.04 inch would be satisfactory. Fuseshaving a voltage rating of 600 volts do not call for caps of increasedwall thickness as long as the current rating thereof is not in excess of400 amps. If the current rating is as high as 600 amps. and the voltagerating as high as 600 volts, the wall thickness should be increased tothe order of 0.07 inch. The copper content of the brass should be atleast in the order of 80 percent, irrespective of the particular voltagerating and current rating.

Referring now to FlG. 11 which applies to the voltage ratings of 250volts and 600 volts, it is apparent from that figure that fuses having acurrent rating of 400 amps.

egin to limit shortcircuit currents if the latter are in excess ofl0,000-1l,000 amperes, and that fuses having a current rating of 600amperes begin to limit short-circuit currents if the latter are inexcess of 16,000 amperes.

lG. 12 shows that fusion occurs in about 0.01 sec. at the occurrence ofshort-circuit currents in the order of 10,000 amperes, and 16,000amperes, respectively.

Fuses according to this invention can be assigned UnderwritersLaboratories current ratings. Whenever the terms current rating, orrated current are being used in this context, these terms have been usedwithin a broader meaning, i.e. with reference to a current which a fusecan carry for an indenite period of time without excessive heating.

ln conventional low-voltage current-limiting high-interruptingcapacityfuses the arc-quenching filler is formed by quartz sand. Because of theextremely high de-ionizing or cooling action of that arc-quenchingmedium, it is in quartz-sand-lled fuses relatively easy to achieve asufficiently elevated arc-voltage for the particular purpose in hand.Where the pulverulent arc-quenching iiller has a smaller de-ionizing andcooling action than quartz sand, the problem Vto achieve a suillcientlyelevated and sullciently stable arc-voltage requires particularconsideration. The geometry of the ribbon type fuse links, and moreparticularly the number and size of their perforations, must becarefully determined if acceptable results, or best results, as regardsarc-voltage and energy dissipation during the interrupting process areto be obtained.

The basic considerations concerning the design of fuse links with a viewof generating the right kind of arcvoltage are outlined below. FIG. 13shows a ribbon fuse link having but one single central perforation. Thisis a simplified version of the fuse links illustrated in FIGS. 1 6. Fuselinks have a given length, a given width and a given thickness and theseparameters determine primarily the current carrying capacity of a fuselink in a `given pulverulent arc-quenching iiller. ln other words, thecurrent-carryingcapacity, or current rating, changes when the link issubmersed in another arc-quenching filler. The specific heat of themetal of which the link is made and its latent heat of fusion togetherwith the minimal cross-section q of the fuse link determine the meltingft2-dt or fusing ft2-dt and, therefore, also the currents required tocause fusion within given times not exceeding 0.01 sec. Thus the conceptof current-limiting ratio can readily be translated into terms ofdimensions of a fuse link, or terms of fuse link geometry.

The fuse link of FlG. 13 las a temperature gradient in a directionlongitudinally of the link and a temperature gradient in transversedirection. The temperature within the plane of minimal cross-section ishighest at the points T and smallest at the points t. Hence fusion isinitiated at the points T and progresses toward the points t. Thus thelink is initially severed by an incision progressively expanding frompoints T to points t. The result of this initial severing process is theformation of a short arc gap which may be in the order of 1/0 inch. Uponformation of that gap the period of back-burning begins, i.e. the periodof gap growth in a direction longitudinally of the link. The arc voltagegenerated during the period of back-burning, or longitudinal gap growth,depends largely upon the diameter of the perforation or perforations ofthe link, as will become more apparent from the ensuing analysis of thisphenomenon.

FIGS. 14 and l5 illustrate two links which are identical, except for thegeometry and size of their center perorations. ln both fuse links theminimal cross-section q is equal and, therefore, the fusing fiZ-a't isequal in both instances. The same minimal cross-section has beenobtained, in one instance, by providing one single center perforationhaving a relatively large diameter, and in the other instance, byproviding a pair of center perforations having a relatively smalldiameter. The difference of two extreme typical designs in terms of arcvoltage can be explained as follows:

The higher the current density in an electric arc, the higher thevoltage drop across the arc or the are voltage. The higher the voltagedrop across an arc, or the arc voltage, the higher the speed of burnoackfrom the relatively hot point of arc initiation to areas where thepulverulent arc-quenching filler is relatively cool. The higher thespeed of burnback, the greater the stability of the arc voltage, ascontrasted with an arc voltage which decays rapidly upon having reachedan initial peak. Stable arc voltages tend to accelerate the decay of thecurrent to zero and thus to reduce the arcing fig-dt as well as thetransient pressure within the casing oi the fuse.

Leaving aside magnetic eld action upon current density, it is apparentfrom FTGS. 14 and l5 that the current densities at the time ci arcinitiation will be about the same in both instances since the minimalcrosssection q is the same in both instances. ln the case of FIG. l5 thecurrent density will drop to a minimum upon a short burnhack equal tothe relatively small diameter d of each of the two perlorations in thelink. The aforementioned minimum current density largely depends uponthe ratio of the cross-section of the iuse link to the instantaneouscurrent being carried by it. lt is apparent from 14 referring to a linkhaving one single rela tively large perforation whose diameter is D thatin this case the current density does not decay to its minimum valueupon as short a burnback as in the case or FIG. l5. ln other words,perforations of relatively large size tend to maintain the arc voltageat a relatively high evel. Thus it is in thinterest oi minimizing thearcing ,lf2-dt as well as the transient pressure to provide relativelylarge pertorations in the fuse link. lf the size of the pertorations isexcessive this tends, however, to reduce the currentcarryingcapacity ofthe link. Thus the size or diameter of the perforations must bedetermined by striking a judicious compromise between arc voltage andpressure equirements, on the one hand, and current-carrying-capacityrequirements, on the other hand.

The current trace illustrated in 16a and the voltage trace illustratedin FIG. 161i) show that fuses embodying the present invention have acurrent-limiting action, and that the latter is quite different from nonmally encountered in current-limiting high-interruptingcapacity fuseshaving fulgurite-forming arc-quenching quartz fillers therein. Thevoltage trace illustrated in FIG. 16a is characterized by a relativelysmall rise of the arc voltage at the time t1 of arc inception. Theinitial arc voltage rises to less than 1.5 times circuit voltage. Duringa large portion of the arcing time the arc voltage rises while thecurrent gradually decays to zero. This rise occurs immediately alter t1.After reaching a peak the arc voltage decreases. The current becomeszero at t2 and at t2 the voltage across the fuse is equal to the systemvoltage. The initial decrease of current intensity, i.e. the decrease incurrent intensity immediately following the time of arc inception t1 isrelatively small and thereafter the current decays approximately at aconstant rate. There is no signiiicant rise of tne voltage across thefuse in the interval of time between fault inception at to and arcinception at t1.

The lack of formation of a high voltage spike immediately upon arcinception at t1 may mainly be explained by the fact that the heatabsorbing capacity of gas-evolving arc-quenching nllers such as gypsumis less than the heat absorbing capacity of quartz sand. The voltagegradient across an arc gap formed in quartz sand drops very rapidly asarcing continues and such a rapid decay of the voltage gradient does notseem to occur in current-limiting fuses having a gas-evolvingnon-fulguriteforming arc-quenching filler. Thus the relatively greatstability of the arc voltage may be explained, at least in a large part,by the characteristics of the arc-quenching filler. he overlays db and4b may have some slight delaying action in regard to fusion undershort-circuit conditions, causing fusion to occur at the center lines ofperforations shortly after fusion has occurred at the axially outerlines of perforations. Another factor tending to stabilize the arcvoltage is the judicious selection oi the right diameter of theindividual periorations of the fuse links, as explained above more indetail.

The relatively great uniformity and stability of the arc voltage makesit possible to keep the arcing figur within relatively narrow limits andthus to compensate for the tendency to generate relatively high pressureas a result or" the presence of arc-quenching lillers that evolverelatively large amounts oi gas under the heat of arcs. The limitationof the melting fizdt and of the arcing f2'dt make it possible to useinexpensive casings having a relatively limited rnechanical strength andto close the samek in a substantially gas-tight fashion by relativelylight, or unsubstantial, caps or ferrules of a non-ferrous metal,preferably red brass, thus drastically reducing the watt lossesoccurring in such fuse structures.

Having disclosed preferred embodiments of my invention, it is desiredthat the same be not limited to the particular structure disclosed. itwill be obvious to any person skilled in ti e art that manymodifications and changes may be made without departing from the broadspirit and scope of the invention as defined by the following claims.

I claim as my invention:

l. An Underwriters Laboratories standard size lowvoltage fusecomprising:

a tubular casing of a homogeneous organic insulating material;

a pulverulent non-fulgurite-forming gas-evolving arcquenching fillerinside said casing;

a pair of blade contacts extending in a direction longitudinally of saidcasing each projection from the outside of said casing into the insidethereof;

ribbon fuse link means of copper submersed in said iller conductivelyinterconnecting the axially inner ends of said pair orV blade contacts,said fuse link means having a plurality of transverse lines ofperforations forming serially related zones of reduced cross-sectionsufficiently small to result in a currentlimiting ratio of less than 30,and said fuse link means having dimensions adapted to impart to saidfuse a current rating of at least 400 amperes;

a loW-iusing-point linkevering metal overlay on said fuse link means;

a pair of terminal caps each delining a passage for one of said pair ofblade contacts and each closing one end of said casing sufii ientlytight to substantially preclude the emission of hot products of arcingtherefrom, and said pair of caps consisting of a non-ierrous metal.

2. An Underwriters Laboratories standard size lowvoltage fusecomprising:

a tubular casing of a homogeneous organic insulating Y material;

a pulverulent non-fulgurite-orrning gas-evolving arcquenching iillerinside said casing; Y

a pair oi blade contacts extending in a direction longitudinally of saidcasing each projecting from the outside of said casing into the insidethereof;

ribbon fuse link means oi copper submersed in said filler conductivelyinterconnecting the axially inner ends of said pair of blade contacts,said fuse link means having a plurality of transverse lines ofperforations forming serially related zones of reduced cross-sectionsuiiciently small to result in a currentlimiting ratio of less than 3i),and said pair of fuse link means having dimensions adapted to impart tosaid fuse a current rating of at least 400 amperes;

a low-fusing-point link-severing metal overlay on said fuse link means;

a pair of terminal caps each delining a passage for one of said pair ofblade contacts and each closing one end of said casing suliicientlytight to substantially preclude the emission of hot products of arcingtherefrom, and said pair of caps consisting of brass.

3. An Underwriters Laboratories standard size lowvoltage fusecomprising:

a tubular casing of a homogeneous organic insulating material;

a pulverulent non-fulgurite-forming gas-evolving arcquenching fillerinside said casing;

a pair of blade contacts extending in a direction longitudinally of saidcasing each projecting from` the outside of said casing into the insidethereof;

ribbon fuse link means of copper submersed in said filler conductivelyinterconnecting the axially inner ends of said pair of blade contacts,said fuse link means having a plurality of transverse lines ofperforations forming serially related zones of reduced cross-sectionsufficiently small to result in a currentlimiting ratio or less than 30,and said fuse link vmeans having dimensions adapted to impart to saidfuse a current rating of at least 400 amperes, and the number of saidzones of reduced cross-section and the geometry of the constituentperforations of said lines of perforations determining an averageclearing ratio of less than 5;

a low-fusing-point link-severing metal overlay on fuse link means;

a pair of terminal caps each defining a passage for one of said pair ofblade contacts and each closing one end of said casing suiciently tightto substantially preclude the emission of not products of arcingtherefrom, and said pair of terminal caps consisting of a non-ferrousmetal. Y

4. A one-time fuse having a voltage rating of 250 volts comprising:

a tubular casing of liber having a length of approximately 8% inches, adiameter of less than 2.406 inches and a wall thickness of at least 1Ainch.

a pulverulent non-iulgurite-forming gas-,evolving arcquenching fillerinside said casing; Y

a pair of blade contacts extending in a direction longitudinally of saidcasing each projecting from the outside of said casing into tieinside'thereof;

ribbon fuse link means of copper submersed in said filler conductivelyinterconnecting the axially inner ends of said pair of blade contacts,said link means being adapted to impart to said fuse a current rating of400 amperes, said fuse link means having three transverse lines ofcircular perforations forming serially related zones of reducedcross-sectional area, said zones being suihcently small to result in acurrent-limiting ratio of less than 30 and the diameter of theconstituent perforations of said lines of perforations determining anaverage clearing ratio of less than 5;

an overlayor a low-fusing-point link-severing metal arranged on saidfuse link means immediately adjacent the center line of said transverselines of perforations;

a pair of terminal caps each deiining a passage for one of said pair ofblade contacts and each closing one end of said casing suiiciently tightto substantially preclude the emission of hot products or arcingtherefrom, and said pair of caps consi ing of brass.

said

an overlay of a loW-fusing-point link severing metal arranged on saidfuse link means immediately ad- ]acent the center line of said lines ofperforations;

mately 10% inches, a diameter of less than 2.906 inches and a wallthickness of at least 3/16 inch,

a pulver-nient non-fulgurite-forrning gas-evolving arcquenching iillerinside said casing;

a pair of blade contacts extending in a direction lontudinally of saidcasing each projecting from the outside of said casing into the insidethereof;

ribbon fuse link means of copper submersed in said iiller conductivelyinterconnecting the axially inner ends of said pair of blade contacts,said fuse link means being adapted to impart to said -f-use a currentrating of 400 amperes, said fuse link means having five transverse linesof circular perforations forming serially related zones of reducedcross-sectional area, said zones being sufficiently small to result in acurrent-limiting ratio of less than 30, and the diameter of theconstituent perforations of said lines of pertorations determining anaverage clearing ratio of less than 5;

a pair of terminal caps each defining a passage for 5 one said pair ofblade contacts and each closing one end of said casing suiiciently tightto substantially preclude the emission of hot products of arcin-gtherefrom, and said pair of caps consisting of brass.

7. A one-time fuse having a voltage rating of 600 volts,

a tubular casing of tiber having a length of approxin mately 13S/8inches, a diameter of less than 3.419 inches and a Wall thickness of atleast 1A inch;

a pulverulent nonaf-ultgurite-forming gas-evolving arcquenching llerinside said casing;

rating of 600 amperes, said fuse link means havl5 a pair of bladecontacts extending in a direction loning three transverse lines ofcircular perforations gitudinally of said casing each projecting fromthe forming serially related zones of reduced cross-secoutside of saidcasing into `the inside thereof; tional area, said zones beingsufciently small to ribbon fuse link means of copper submersed in saidresult -in a current-limiting ratio of less than 30I and rlerconductively interconnecting the axially inner the diameter of theconstituent perforations of said ends of Said pair of blade contacts,said fuse link lines of perforations determining an average clearingmeans being adapted to impart to said -fuse a current ratio of less than5; rating of 600 amperes, said ruse link means having an overlay of alow-fusing-point link severing metal ve transverse lines of perorationsforming searranged on said fuse link means immediately adrially relatedZones of reduced cross-sectional area, jacent the center line of saidlines of perforations; Said ZOnes being suiiciently small to result in acura pair of terminal caps each deining a passage for rent-limitingratio of less than 30, and the diameter one of said pair of bladecontacts and each closing of the constituent perforations of said linesof perone end of said casing sufficiently tight to substan- `fOratiOnsdetermining an average clearing ratio of tially preclude the emission ofhot products of arcing -less than 5; therefrom, and said pair of capsconsisting of brass. an overlay of a loW-fusing-point link severingmetal 6. A one-time fuse having a voltage rating of 600 arranged 0n saidfuse link means immediately advolts comprising: jacent the center lineof said lines of perforations; a tubular casing of fiber having a lengthof approxi- 'f1 P311' 0f tfmmfll CaPS each deing a Passage `OI mately11S/s inches, a diameter of less than 2.906 3f one 0f Sad Pa.1I 0fPiaf-le COQHCS and Cach Closing inches and a Wall thickness of at least9%16 inch, o 0F15 end of Sad Camlg Suclly tight t0 Substana pulverulentnon-.fulgurite-forming gas-evolving arcliyfpcludedthe.mlsilonfof hotprofluts d anfmg quenching uer inside Said casing; ere rom, an sai pairo caps consisting of biass. a Plil'dff llladef couacts extendg in atfecon ll' 40 References Cited in the le of this patent gitu ma y o saicaslng eac proyec ing rom e outside of said casing into the insidethereof; UNITED STATES PATENTS ribbon fuse link means of coppersubmersed in said 2582x587 Burt et al- June 29, 1954 ller conductivelyinterconnecting axially inner 283268 Kozaclfa A1913 29, 1958 ends ofsaid pair of blade contacts, said fuse link redeflck Maf 3, 1959 meansbeing adapted to impart to said `ruse a current 2:988620 Kg OTHERREFERENCES Jacobs: Current Limiting Fuses: Their Characteristics andOperations, AIEE Transactions, Part III, vol. 75,

1956, pages 988-993.

1. AN UNDERWRITERS'' LABORATORIES STANDARD SIZE LOWVOLTAGE FUSECOMPRISING: A TUBULAR CASING OF A HOMOGENEOUS ORGANIC INSULATINGMATERIAL; A PULVERULENT NON-FULGURITE-FORMING GAS-EVOLVING ARCQUENCHINGFILLER INSIDE SAID CASING; A PAIR OF BLADE CONTACTS EXTENDING IN ADIRECTION LONGITUDINALLY OF SAID CASING EACH PROJECTION FROM THE OUTSIDEOF SAID CASING INTO THE INSIDE THEREOF; RIBBON FUSE LINK MEANS OF COPPERSUBMERGED IN SAID FILLER CONDUCTIVITY INTERCONNECTING THE AXIALLY INNERENDS OF SAID PAIR OF BLADE CONTACTS, SAID FUSE LINK MEANS HAVING APLURALITY OF TRANSVERSE LINES OF PERFORATIONS FORMING SERIALLY RELATEDZONES OF REDUCED CROSS-SECTION SUFFICIENTLY SMALL TO RESULT IN ACURRENTLIMITING RATIO OF LESS THAN 30, AND SAID FUSE LINK MEANS HAVINGDIMENSIONS ADAPTED TO IMPART TO SAID FUSE A CURRENT RATING OF AT LEAST400 AMPERES; A LOW-FUSING POINT LINK-SEVERING METAL OVERLAY ON SAID FUSELINK MEANS; A PAIR OF TERMINAL CAPS EACH DEFINING A PASSAGE FOR ONE OFSAID PAIR OF BLADE CONTACTS AND EACH CLOSING ONE END OF SAID CASINGSUFFICIENTLY TIGHT TO SUBSTANTIALLY PERCLUDE THE EMISSION OF HOTPRODUCTS OF ARCING THEREFROM, AND SAID PAIR OF CAPS CONSISTING OF ANON-FERROUS METAL.